A soluble low Km form of cyclic 3':5' -guanosine monophosphate phosphodiesterase (EC 3.4.1.17), which appears to be under allosteric control, has been partially purified from bovine liver homogenates. The preferred substrate for the enzyme is cGMP (Km values: cGMp less than cIMP less than cAMP). The enzyme has a molecular weight of 360,000 daltons determined by gel filtration and a pI of 5.l8 determined by isoelectric focusing. The cAMP-hydrolyzing activity is stimulated more than 20- fold by micromolar concentrations of cGMP. p-(Hydroxymercuri)-benzoic acid abolishes this positive cooperativity, presumably by oxidizing sulfhydryl groups on the enzyme essential for allosteric effects.